Wikiosity
http://www.maxtility.com/scgi-bin/wikiosity.cgi

This is useful if your students use Wikipedia often do find quick information. I can see using this with my 6th graders especially, but it would also apply to middle schoolers in general. You know, their searching skills aren't exactly perfected yet, but this gives them great direction on how to find key topics within the subject we're studying.